This invention relates generally to apparatus and methods for polarizing light and particularly to fiber optic apparatus and methods for providing light of a predetermined polarization.
It is well known that in many fiber optic systems, it may be desirable to have light of a known polarization state at selected points for input to components whose operation is polarization dependent in order to minimize errors. The state of polarization is particularly important in a device such as an optical fiber rotation sensor. In a polarized optical fiber rotation sensing system, drift errors due to changes in polarization are determined by the quality of the polarizer.
A linear polarization state in a fiber optic rotation sensor is typically achieved with some type of linear polarizer such as the fiber optic polarizer described in U.S. Pat. No. 4,386,822 to Bergh. The polarization state input to the polarizer is arbitrary in general. The polarizer couples light of undesired polarizations out of the fiber and permits light having only a selected desired polarization to propagate through the fiber. Bergh discloses a fiber optic polarizer including a length of optical fiber mounted in a curved groove in a quartz substrate. The substrate and a portion of the optical fiber are ground and polished to remove a portion of the cladding from the fiber to form an interaction region. The portion of the fiber in the groove is convexly curved as viewed looking toward the polished surface. The birefringent crystal mounted on the substrate over the interaction region is in close proximity to the core of the fiber optic material. The crystal is positioned to partially intersect the path of light propagating in the optical fiber so that evanescent field coupling couples light of undesired polarizations from the optical fiber into the crystal.
The birefringent crystal has different wave velocities for waves of different polarizations. With polarizations for which the wave velocity in the crystal is less than the wave velocity in the optical fiber, the light carried by the optical fiber excites a bulk wave in the crystal, which causes light to escape from the optical fiber into the crystal. No bulk wave is excited in the crystal for polarizations having wave velocities in the crystal greater than in the fiber so that light having such polarizations remains guided within the optical fiber. The indices of refraction of the crystal are such that a wave having a polarization along one of the principle axes of the crystal will propagate more slowly in the crystal than the optical fiber; and a wave having a polarization along a second principle axis will propagate at a greater speed in the crystal than in the optical fiber.
An improved apparatus for producing light of a known polarization includes a polarization controller placed in the fiber between the light source and the polarizer with the polarization controller being adjusted to provide light of a desired polarization for input into the polarizer. However, in a typical system the polarization state input to the polarization controller varies due to the environmental sensitivities of the optical fiber. Variations in temperature and pressure, vibrations, and aging of the materials may cause significant changes in the polarization output from the polarization controller to the polarizer. Therefore, in a system which includes a polarization controller fixed to vary the polarization of light input by a predetermined amount, the time varying polarization of the light input to the polarization controller causes signal fading.
Other problems associated with prior polarizers are that naturally occurring crystals may not have the desired indices of refraction since each naturally occurring crystal has only certain indices of refraction determined by the crystalline structure, and it is difficult to adequately bond the crystal to the substrate. The crystals are not environmentally stable, which is another source of error.
A second improved polarizer includes a feedback system for detecting the light coupled out of the fiber, which represents an error signal. The error signal is used to adjust the polarization of the light input to the polarizer to minimize the amount of light coupled out of the system.
The effectiveness of such polarizers is highly dependent upon the index of refraction of the crystal. Unfortunately, the indices of refraction of birefringent crystals are highly temperature-sensitive. In order to be suitable for inertial guidance applications, a rotation sensor must have a high degree of temperature stability because such rotation sensors must be operable over a range of temperature from -65 degrees Celsius to +85 degrees Celsius according to standard military specifications. Such temperature extremes might be experienced by aircraft and missiles while in operation between low altitudes and high altitudes.